This invention relates to a hot runner mold for runnerless molding.
In the injection molding, a fluid passageway such as a runner or sprue has hitherto been mechanically designed to conduct molten plastics mass into a mold. Each time a plastics product is molded, it is taken out in a state accompanied with unnecessary mass or refuse left in said fluid passageway. An attempt to regenerate such runner refuse would lead to economic disadvantages and deteriorate the quality of a resultant product. Therefore, the runner refuse has been simply wasted, resulting in the loss of raw plastics material and, in an extreme case, acting as a source of public nuisance. Further, manufacture of a plastics product with unnecessary runner refuse attached thereto prolongs the cyclic period of molding and requires aftertreatment, thus obstructing the saving of work.
To date, therefore, various types of molds have been devised. Particularly in recent years, numerous types of hot runner mold have been developed.
With the conventional hot runner mold, a spacer is arranged between a fixed head die plate and fixed die block to provide a free space, in which a hot runner block is received. The runner block is bored with a runner hole and heated by a heater so as to maintain plastics material passing through the runner hole in a molten state throughout the cyclic period of injection molding.
However, the known hot runner mold used a large runner block, prolonging the time required for the runner block to be heated to a prescribed temperature, making it necessary to apply a large capacity heater, rendering a mold itself bulky and presetting difficulties in applying uniform heating to the runner block and controlling the temperature thereof. It has been difficult to construct such mold as suppresses heat transfer from the runner block to other mold parts and also to an injection molding machine and prevents its own deformation resulting from thermal stresses applied thereto.